Preparetion And Study Of Langasite Thin Films

Surface acoustic wave sensors have been widely studied and applied to temperature sensing, pressure sensing, gas sensing and other fields for their advantages such as easy miniaturization, high radiation resistance and high reliability. In order to make surface acoustic wave sensors work properly in high-temperature environment, piezoelectric material with good stability in high temperature and chemical stability was needed. Langasite(La3Ga5SiO14, LGS) is a new type of piezoelectric material with excellent properties, The electromechanical coupling coefficient of LGS is 2~3 times as large as quartz’s, LGS also exhibts stability in high temperature and chemical stability, and has no phase transition from room temperature to melting point(1470 ℃). Along with the development of miniaturization and integration of devices, the membranization of piezoelectric material was stimulated to have a new progress. Thus, preparation of LGS thin films by sol-gel method in this paper is of great significance for the research of new high temperature thin films surface acoustic wave sensors.Firstly, the preparation of LGS thin films was studied preliminarily in this paper by using sol-gel method, LGS thin films were deposited on silicon and sapphire substrate respectively by using the same process conditions. LGS thin films will crystallize only when annealing temperature is higher than 1100 ℃, SiO2 on silicon substrate will recrystallize at this temperature, which illustrates that silicon substrate is not applicable for preparation of LGS thin films, meanwhile, LGS thin films with accurate stoichiometric ratio was prepared on sapphire substrate successfully.Secondly, the factors, such as heat temperature and time, citric acid addition and drying temperature, which influence on the quality of LGS thin films prepared on sapphire substrate were studied. The results show that the lowest crystallization temperature of LGS thin films is 1100 ℃ and the optimum annealing temperature is 1200 ℃. After heating more than 3 h, the degree of crystallinity of the LGS films does not change. The crystallinity of LGS thin films is best when the citric acid addition is 2 times of the total amount of La(NO3)3·6H2O and Ga(NO3)3. Drying temperature has a great impact on the quality of films. A crossing heat process(drying at 200 ℃ for 1 h + drying at 300 ℃ for 1 h + …) has been proposed in this paper to prepare high(110) orientated and dense LGS thin films.Finally, based on crossing heat process, the influence of the concentration of the sol on the grain size and RMS of LGS thin films was studied, the results show that the grain size increases with the increase of the concentration of the sol, the RMS of 0.1 mol/L, 0.2 mol/L and 0.3 mol/L sol are 11.8 nm, 28.2 nm and 35.4 nm, respectively. The optimum process of preparing LGS thin films was determined finally, the sol concentration is 0.1 mol/L; the citric acid addition is 2 times of the total amount of La(NO3)3·6H2O and Ga(NO3)3; crossing heat process is used for drying process; the temperature and time of high temperature treatment are 1200 ℃ and 3 h, respectively. By using this optimal process, using ethylene glycol as solvent, LGS thin films were prepared by sol-gel method. The results show that the prepared LGS films do not have preferred orientation, and the films are not dense, which illustrates that the optimal process is only suitable for the sol-gel method with ethanol as solvent. The optimal process for the sol-gel method with ethylene glycol as solvent still need to be studied further in the future.